Update: Niantic has revealed its plans for the worldwide release of Mewtwo. The legendary Pokémon will be available through "Exclusive Raid Battles" requiring players to be invited before they can participate.
To receive an invite, players must have completed a normal Raid at the gym where the Exclusive Raid will take place. When the invite is sent, they'll be given advance notice of when and where they'll be able to challenge Mewtwo.
Mewtwo will arrive in the game "in the coming weeks".
Also, all four of the legendary bird Pokémon - Lugia, Articuno, Zapdos and Moltres - will be available at raids from now (14 August) until 31 August.
Original Story: Mewtwo has finally been made available in smash hit mobile game Pokémon Go after developer Niantic opened up co-operative raid battles against the series' most iconic legendary beastie in Japan.
The 150th Pokémon appeared at the conclusion of the Pikachu Outbreak Festival event today (14 August) in Yokohama, which focused on a parade through the city featuring dozens of people dressed as Pikachu.
It also marked a Pokémon Go Stadium event, at which players teamed up to take down powerful Pokémon which they then attempt to capture.
Niantic released the first legendary Pokémon, Lugia and Articuno, into the game last month with Moltres and Zapdos following. Most assumed this event would end with the release of Ho-Oh the fifth and final legendary bird.
Mewtwo was the most powerful Pokémon players could attempt to capture in the original Red and Blue (and Green in Japan) versions of the very first Pokémon title. It appeared at the end of the game, after players completed the story, as a sort-of final fight.
In game screenshots have revealed that Mewtwo has a CP (combat points) rating of around 49,000, making it the most powerful Pokémon players can take on in Raids. If captured, it has a CP of around 2200.
It's not clear when Mewtwo will be made available around the world, or whether this is a one-off event ahead of a larger, global-scale event.
Last month, the release of those first legendary Pokémon was made at a real-world event held in Chicago - Pokémon Go Fest - which then lead to a release around the world. The event certainly had its problems.
As in Chicago, attendees were given a special code to access to the Raid, and reportedly the Raid comes with a 100% capture rate, meaning everyone in attendance will get a Mewtwo.